Biological systems are extremely complex. Understanding them requires the ability to see detailed relationships between structure, function, and localization at various levels of resolution. A correlative approach, which uses both optical microscopy and electron microscopy produces the most comprehensive results. For example, light microscopy information can be used to identify areas of biological importance and their dynamics within a sample, and then electron microscopy can be used to resolve structural details within those areas after fixation and/or staining.
One powerful technique that has gained wide acceptance for research into cellular structure, molecular dynamics, and motility is the application of recombinant genetic techniques to link “reporter” genes to “genes of interest” (GoIs). Thus, when a particular GoI is expressed during normal genetic transcription/translation processes, the reporter gene will also be expressed, producing a small, detectable protein which ends up attached to the “protein of interest” (PoI) encoded for by the GoI.
A widely-accepted reporter gene is that encoding for a green fluorescent protein (GFP), these reporter genes being available in wild-type and genetically-modified versions, and the expressed GFPs having fluorescence that extends from blue to red in emission wavelengths. The GFP is relatively small (29.6 kDa, 3 nm in diameter by 4 nm long) with its chromophore well protected inside and not requiring any co-factors for light emission. In biological environments, all that is needed to “light up” a GFP is illumination by a laser at an appropriate wavelength for GFP excitation and fluorescent decay.
Clearly, if the X-Y-Z location of the GFP can be determined precisely within a cell (say, to 10 nm accuracy) then the location of the PoI would be known to a similar accuracy. The fluorescing GFP can be observed through a light microscope and so the location of the PoI can be seen relative to observable structures in the sample. An electron microscope, although unable to see the fluorescense, can then be used to form an image of the structure at a much higher magnification than is possible in a light microscope. Several techniques in the prior art have been proposed and, in some cases, demonstrated, for achieving high positional information from various fluorescent markers such as GFPs, dyes, and quantum dots. In one technique, proposed in U.S. Pat. No. 7,317,515 to Buijsse et al. for “Method of Localizing Fluorescent Markers,” which is assigned to the assignee of the present application and which is hereby incorporated by reference, a charged particle beam would scan the surface of the sample, damaging the markers and extinguishing the fluorescence when the beam hits the fluorescent marker. The location of the fluorescent marker would then correspond to the position of the charged particle beam when the fluorescence is extinguished. Because the charged particle beam can be focused to a much smaller point than the laser that illuminates the marker, and the position of the charged particle beam at any time during its scan would be determined with great accuracy, the position of the fluorescent marker, and therefore the position of the PoI, would then be determined with similar accuracy.